Sub‐2 nm IrRuNiMoCo High‐Entropy Alloy with Iridium‐Rich Medium‐Entropy Oxide Shell to Boost Acidic Oxygen Evolution

Abstract Ensuring high catalytic activity and durability at low iridium (Ir)usage is still a big challenge for the development of electrocatalysts toward oxygen evolution reaction (OER) in proton exchange membrane water electrolysis (PEMWE). Here, a rapid liquid‐reduction combined with surface galvanic replacement strategy is reported to synthesize the sub 2 nm high‐entropy alloy (HEA) nanoparticles featured with Ir‐rich IrRuNiMo medium‐entropy oxide shell (Ir‐MEO) and a IrRuCoNiMo HEA core (HEA@Ir‐MEO). Advanced spectroscopies reveal that the Ir‐rich MEO shell inhibits the severe structural evolution of transition metals upon the OER, thus guaranteeing the structural stability. In situ differential electrochemical mass spectrometry, activation energy analysis and theoretical calculations unveil that the OER on HEA@Ir‐MEO follows an adsorbate evolution mechanism pathway, where the energy barrier of rate‐determining step is substantially lowered. The optimized catalyst delivers the excellent performance (1.85 V/3.0 A cm −2 @80 °C), long‐term stability (>500 h@1.0 Acm −2 ), and low energy consumption (3.98 kWh Nm −3 H 2 @1.0 A cm −2 ) in PEMWE with low Ir usage of ≈0.4 mg cm −2 , realizing the dramatical reduction of hydrogen (H 2 ) production cost to 0.88 dollar per kg (H 2 ).